1. Field of the Invention
The present invention relates to electronic circuits, and more particularly to temperature sensors for producing temperature-dependent electronic signals.
2. Description of Related Art
Temperature sensors such as digital thermometers often measure temperature by exploiting the thermal-voltage characteristics of a diode. The temperature coefficient of a diode is the voltage drop across the diode as a finction of temperature while the diode is forward-biased by a constant current. The temperature coefficient of a diode tends to be a constant that decreases as temperature increases. A typical temperature coefficient is about -2 mV/.degree. C. Furthermore, the voltage drop across a diode at absolute zero tends to be the bandgap voltage of the diode material, which is reliable and traceable to fundamental physical concepts. The bandgap voltage for silicon is about 1.2V. Therefore, the temperature of a diode can be accurately determined by measuring the voltage drop across the diode once the temperature coefficient is known.
Temperature sensors normally generate an output signal that increases with increasing temperature. Therefore, the voltage drop across a diode does not provide a suitable output signal since it decreases with increasing temperature.
A temperature sensor that is known in the art generates voltage drops of two diodes with different current densities. This can be accomplished by applying the same current to different sized diodes, or by applying different currents to identically sized diodes. In either case, the diode with the higher current density exhibits a smaller temperature coefficient (with a smaller absolute value) than the other diode. The diodes, however, have the same voltage drop at absolute zero, namely the band gap voltage of the diode material. Therefore, as temperature increases, the difference between the voltage drops of the diodes linearly increases. The temperature sensor also includes a differential amplifier that receives the voltage drops of the diodes and generates an output signal representing the difference between the voltage drops. The output signal linearly increases as temperature increases. In addition, the output signal is offset and amplified to indicate the temperature as degrees Celsius or Fahrenheit.
Temperature sensors are used in a variety of applications. For instance, a temperature sensor has been used to turn off a microprocessor when the temperature sensor determines that a predetermined temperature is exceeded in order to prevent data errors or reliability problems.
Microprocessors are usually implemented in CMOS technology because of its low DC power dissipation, high noise margin, wide temperature and voltage ranges, overall circuit simplification, layout ease, and high packing density. Temperature sensors, on the other hand, are typically implemented in bipolar technology using diode-connected bipolar transistors to provide the temperature coefficient as base-emitter voltage (V.sub.BE) as a function of temperature. Although BiCMOS technology makes the integration of CMOS and bipolar technologies feasible, the tradeoff between process complexity and device quality may be unacceptable for microprocessors. As a result, typically the temperature sensor and the microprocessor are manufactured as separate integrated circuit chips, implemented in separate technologies, and the temperature sensor is placed about 1-2 inches from the microprocessor due to packaging constraints.
A major drawback of this approach is that the temperature sensor senses the temperature of itself, not the microprocessor. The temperature of the temperature sensor is affected by the ambient and/or other nearby circuits besides the microprocessor. Consequently, the temperature sensor senses a temperature that is only loosely correlated with the microprocessor temperature. Another drawback is the cost associated with multiple integrated circuit chips.
Accordingly, a need exists for an improved temperature sensor that senses a microprocessor temperature more accurately and cost-effectively than conventional approaches.